Thomas S. Hooyer

A ring-shear device for the study of till deformation: Tests on tills with contrasting clay contents

Abstract Deformation of subglacial sediment may have had a profound influence on the geomorphic effects and dynamics of Pleistocene ice sheets, but data that bear on the rheology of such sediment are few and contradictory. A means of studying systematically the mechanical properties of glacial tills is provided by a ring-shear device that shears a large annular specimen to high strains at variable rates. Special features of the device allow continuous observation of the distribution of shear strain, measurement of local stresses normal to the direction of shear, and isolation of wall effects. Thus far, a linear-viscous putty and two tills with different clay contents (4 and 32% by weight) have been tested. The experiments with till indicate, as expected, that the presence of dispersed clay minerals reduces markedly both the residual strength and hydraulic diffusivity of till. Clay also reduces spatial variations in normal stress associated with grain bridges. The walls bounding the specimen support only 9–17% of the total resistance to shearing. Strain was localized in both tills, but not in the linear-viscous putty. Strain localization in the tills indicates that their theology departs significantly from linear- and Bingham-viscous models.

Clast-fabric development in a shearing granular material: Implications for subglacial till and fault gouge

Elongate clasts in subglacial till and in fault gouge align during shearing, but the relation between clast-fabric strength and cumulative shear strain for such materials is effectively unknown. This relation was explored in experiments with a large ring-shear device in which a till and a viscous putty that contained isolated clasts were sheared to high strains. As expected, rotation of clasts in the putty is closely approximated by the theory of G.B. Jeffery, who derived the orbits of rigid ellipsoids in a slowly shearing fluid. Clast rotation in the till, however, is strikingly different. Rather than orbiting through the shear plane as predicted by Jeffery, most clasts rotate into the shear plane and remain there, resulting in strong fabrics regardless of the aspect ratios and initial orientations of clasts. This divergent behavior is likely due to slip of the till matrix along the surfaces of clasts, which is a natural expectation in a granular material but violates the no-slip condition of Jeffery9s model. These results do not support the widespread belief that subglacial till deformation results in weak clast fabrics. Thus, many tills with weak fabrics thought to have been sheared subglacially to high strains, like many basal tills of the Laurentide Ice Sheet, may have been sheared only slightly with little effect on either ice-sheet dynamics or sediment transport. In addition, these results indicate that in simple shear the rotation of clasts in till and in fault gouge is best analyzed with the model of A. March, who treated inclusions as passive markers.

Soft-bed experiments beneath Engabreen, Norway: Regelation, infiltration, basal slip and bed deformation

To avoid some of the limitations of studying soft-bed processes through boreholes, a prism of simulated till (1.8 m � 1.6 m � 0.45 m) with extensive instrumentation was constructed in a trough blasted in the rock bed of Engabreen, a temperate glacier in Norway. Tunnels there provide access to the bed beneath 213 m of ice. Pore-water pressure was regulated in the prism by pumping water to it. During experiments lasting 7-12 days, the glacier regelated downward into the prism to depths of 50- 80 mm, accreting ice-infiltrated till at rates predicted by theory. During periods of sustained high pore- water pressure (70-100% of overburden), ice commonly slipped over the prism, due to a water layer at the prism surface. Deformation of the prism was activated when this layer thinned to a sub-millimeter thickness. Shear strain in the till was pervasive and decreased with depth. A model of slip by ploughing of ice-infiltrated till across the prism surface accounts for the slip that occurred when effective pressure was sufficiently low or high. Slip at low effective pressures resulted from water-layer thickening that increased non-linearly with decreasing effective pressure. If sufficiently widespread, such slip over soft glacier beds, which involves no viscous deformation resistance, may instigate abrupt increases in glacier velocity.

Diffusive mixing between shearing granular layers: constraints on bed deformation from till contacts

Shearing of subglacial till has been invoked widely as a mechanism of glacier motion and sediment transport, but standard indicators for determining shear strain from the geologic record are not adequate for estimating the very high strains required of the bed-deformation model. Here we describe a laboratory study of mixing between shearing granular layers that allows an upper limit to be placed on bed shear strain in the vicinity of till contacts. Owing to random vertical motions of particles induced by shearing, mixing can be modeled as a linearly diffusive process, and so can be characterized with a single mixing coefficient, D. Ring-shear experiments with equigranular beads and lithologically distinct tills provide the value of D, although in experiments with till D decreases systematically with strain to a minimum value of 0.0045 mm 2 . Kinetic gas theory provides an estimate of the dimensionless mixing coefficient which is within an order of magnitude of laboratory values. Knowing the minimum value of D, the distribution of index lithologies measured across till contacts in the geologic record can be used to estimate the maximum shear strain that has occurred across till contacts. Application of this technique to the contact between the Des Moines and Superior Lobe tills in east-central Minnesota, U.S.A., indicates that shear strain did not exceed 15 000 at the depth of the contact.

Flow mechanism of the Des Moines lobe of the Laurentide ice sheet

Rapid flow of the Des Moines lobe of the Laurentide ice sheet may have been related to its unlithified substrate. New reconstructions of the lobe, based on moraine elevations, sediment subsidence during moraine deposition, and flow-direction indicators, indicate that the lobe may have been ∼3 times thicker than in previous reconstructions. Nevertheless, implied basal shear stresses are < 15 kPa, so internal ice deformation was not significant. Instead, the lobe likely moved by a combination of sliding, plowing of particles through the bed surface, and bed shear. Consolidation tests on basal till yield preconsolidation stresses of 125-300 kPa, so effective normal stresses on the bed were small. A model of sliding and plowing indicates that at such stresses most particles gripped by the ice may have plowed easily through the till bed, resulting in too small a shear traction on the bed to shear it at depth. Consistent with this prediction, measurements of orientations of clasts in basal till yield a weak fabric, implying pervasive bed shear strain less than ∼2, although some stronger fabrics have been reported by others. We infer, tentatively, that movement was principally at the bed surface by plowing.

Assessing subglacial processes from diatom fragmentation patterns

Reconstructing the size and glacial style of past ice-sheet advances requires interpreting complex glacial sedimentary facies. We use diatoms, a major component of Antarctic continental shelf deposits, to infer the physical conditions under which these deposits were emplaced. The degree of diatom fragmentation and the presence of diatoms of varying stratigraphic age in glacial sediments provide means to qualitatively gauge glacial mixing and transport. Here we report an experimentally calibrated index of diatom fragmentation that provides a simple but objective method of assessing the degree of subshearing imparted on marine glacial sedimentary deposits. By using a ring-shear device to subject diatomaceous sediment to stresses comparable to those beneath the Ross ice streams, we quantitatively assess patterns of diatom comminution resulting from compaction and from progressive shear stress. Elongate pennate diatoms are found to break disproportionately to discoid centric diatoms when subjected to shear stress; thus, a simple ratio of unbroken centric to pennate diatoms provides a reliable gauge of past shearing. Comparison of ring-shear results with a suite of previously analyzed sediments that represent a variety of glacial, glacial-marine, and hemipelagic settings of the Ross Sea and subglacial Ross Embayment demonstrates that this index can be employed for estimating relative subglacial stresses in this setting.

Effect of a cold margin on ice flow at the terminus of Storglaciären, Sweden: implications for sediment transport

The cold-based termini of polythermal glaciers are usually assumed to adhere strongly to an immobile substrate and thereby supply significant resistance to the flow of warm-based ice up- glacier. This compressive environment is commonly thought to uplift basal sediment to the surface of the glacier by folding and thrust faulting. We present model and field evidence from the terminus of Storglaci¨ aren, Sweden, showing that the cold margin provides limited resistance to flow from up-glacier. Ice temperatures indicate that basal freezing occurs in this zone at 10 −1 -1 0 −2 ma −1 , but model results indicate that basal motion at rates greater than 1 m a −1 must, nevertheless, persist there for surface and basal velocities to be consistent with measurements. Estimated longitudinal compressive stresses of 20- 25 kPa within the terminus further indicate that basal resistance offered by the cold-based terminus is small. These results indicate that where polythermal glaciers are underlain by unlithified sediments, ice-flow trajectories and sediment transport pathways may be affected by subglacial topography and hydrology more than by the basal thermal regime.